Nested functions

can someone please explain what are nested functions exactly?
I'm writing a program with some functions. some of those functions are used within other functions.
I've done it before with another program that works fine but now I get this error:
"ISO C forbids nested functions"
can it be because I use a certain function within itself? I tried it with a little simple program just to check if it's possible and it worked fine. so I don't get what the problem is.